Television



$111, 13, 192.7 w. WALTON 2,086,833

TELEVIS ION Filed Oct. 50, 1931 4 Sheets-Sheet 1 Fly/0 319;

July 13, 1937.- w WALTON 2,086,833

TELEVISION Filed Oct. 50, 1931 4 Sheets-Sheet 2 G. W. WALTON TELEVISIONFiled Oct. 50, 1951 July 13, 1937."

4 Shets-She et 5 July 13, 1937.v 3. w. WALTON TELEVI S ION File'dpct'.so, 1951 4Sheets-Sheet 4 mmmnMA Ann/mm uuuuvv VUUUUUUUAV VUUU PatentedJuly 13, 1937 UNITED STATES PATENT OFFICE TELEVISION George WilliamWalton, London, England 12 Claims.

This invention relates to methods of synchronizing the scanning devicesused in television and the like, and has for its object the provision ofone or more control frequencies transmitted with the picture currents insuch a manner that the picture received is not affected thereby and thatthe actual picture .transmission and reception may also be simplified.

Hitherto attempts have been made to transmit a control frequencyseparately from the picture currents either as a separate modulation ofa separate intermediate carrier frequency, or in the intervals betweenthe successive pictures or lines of the picture. These methods haveproved to be complicated and not very successful. A further method hasbeen to have a distinct band in the picture so that the picture currentsare in distinct groups which are used for synchronizing. This methodalso is not satisfactory for some scanning time is lost and as thegroups are not regular, details of the phase or the synchronizationthereby causing great unsteadiness in the picture.

According to the present invention one or more sinusoidal controlfrequencies are transmitted with the picture currents, or as amodulation of the picture currents, which control frequencies are usedat the receiver to maintain correct synchronism, phase and the like, thearrangement being, however, such that the actualpicture is thereby in noway disturbed even if the control frequencies are not electricallybalanced out. This is accomplished by choosing such a control frequencythat its effect in a received picture is balanced out in succeedingpictures by reason of the phase changes of the control frequency whichappear in these succeeding pictures. Actually the control frequencyitself need not change phase,.the change being only apparent, as it iscaused through the relation of the picture frequency to the controlfrequency. According to another method use is made of optical filterswhich are so constructed that the scanning operation produces thecontrol frequency as a modulation of the picture currents,

and a similar but opposite filter at the receiver removes the effect ofsuch a frequency from the picture. This method is also useful inproviding means for producing afrequency which can be used fortransmission, since as it is a modulation of the picture currents, theresulting frequency, or frequencies, in the picture currents may betransmitted direct, for instance by wireless, be received and be appliedwithout'rectification to control the intensity of light, which incombination with scanning will reproduce the picture, the filterremoving any effect of the frequency. In this way, in wireless, thetrans-- mission band is made narrow, thus allowing the transmission of apicture having greater detail,

picture disturb the It is to be understood that several differentfrequencies may in this way be applied to the picture currents forspecific purposes: for instance, a frequency of half the picturefrequency, for the automatic framing of the picture a frequencycomparable with the line frequency for' the purpose of exactsynchronization, and a high frequency for the transmission.

Optical stops or apertures may in some cases be used for producing asinusoidal modulation of the picture currents, this being accomplishedby the change of angle or position of a pencil of light passing through,or by the stop or aperture. This method is very'useful where oscillatoryscanning is used and affords also a simple means of balancing out anyuneven brilliancy in the picture caused by the oscillatory scanning.

The filters above mentioned may be very conveniently produced byphotographic means, which besides being very precise, are cheap and canbe produced in quantity by printing.

The invention will now be described with reference to the accompanyingdrawings, it being understood that the scope of the invention is notlimited to the methods and means described and shown for they, are onlygiven by way of example.

Figs. 1-6 are diagrams illustrating the prin-' ciple of the invention,

Fig. 7 is a circuit diagram showing one arrangement for separatingpicture signals from synchronizing signals,

Figs. 8, 9, 9a, 10, 10a, 11, 11m, 12, 12a, 13, 13a, and 14 to 19illustrate diagrammatically various ways in which the invention may beperformed,

Figs. 20 to 22 show diagrammatically various optical systems for usewith the present inven tion,

Figs. 23 to 25 show various forms of optical stop' which may be used inthe arrangements such as are shown in Figs. 20 to 22,

Fig. 26 is a circuit diagram showing a further arrangement forseparating picture signals and synchronizing impulses,

Figs. 27 and 28 show two forms of filter for use with the invention, andFigs. 27a. and 28a show the distribution of shading on the filters ofFigs. 27 and 28 respectively,

Figs. 29, 30, and 31 are wave-form diagrams showing the operation of theinvention,

Fig.32 is a circuit diagram showing a further arrangement for separatingelectrical impulses, suitable for use with a cathode ray tube,

Figs. 33*and 34 are further diagrams illustrating a way of performingthe invention,

Fig. 35 shows a filter disc,

Figs. 36 to 43 show the electrode system 0 Kerr cells suitable for usewith the invention,

Figs. 44, 45, and 4c illustrate diagrammatiure 2 shows the controlfrequency superimposedon a D. C. component. If 1 and 2 are superposedthe result is as shown in Figure 3. If now in the next complete picturethe phase of the current in Figure 2 is reversed, the detail of thepicture shown in Figure 1 combined with the control frequency will be asshown in Figure 4. It will be seen that the curves according to Figures3 and 4 will also correspond tothe light intensity at the receiver, 1.e. looking at a corresponding part of the reproduced pictures, thebrightness indicated by Figure 3 will be seen in the one completepicture and that indicated by Figure 4 in a succeeding picture. As thephase of the control frequency is reversed in the succeeding picture,and as in television succeeding pictures are shown in rapid successionso that the human eye is unable to follow the change of picture, thecontrol frequency cannot be seen, only an average illumination beingapparent. However, as the detail indicated in Figure 1 does not changephase from Figure 2 to Figure 4 it remains positive and therefore itappears in the picture. If the control frequency according to Figure 2modulates the picture current it varies the amplitude of the detailindicated in Figure 1, reproducing lt in one picture as shown in Figure5 and in the next one as shown in'Figure 6. No control frequency isapparent in the reproduced picture as only the mean value of theimpulses,

shown in Figures 5 and 6 is seen; the average value of the picturecurrent obviously changes sinusoidally at the control frequency as shownin Figure 2. Where the control frequency is only added to the picturecurrent as in Figures 3 and 4, the D. C. component shown in Figure 2,al-

though preferred, is not specifically required.

At the receiver the picture currents may be.

passed through the primary of a transformer, or an inductance, such asthe coils of the synchronizing device. In the case of a controlfrequency used for synchronizing or phasing, since the picturefrequencies are in general higher, they are to.a large extent chokedback from the synchronizing or phasing coils. This selective action maybe improved by the use of a condenser I offering low impedance to thepicture frequencies but high impedance to the control frequencies. Apreferred arrangement is shown in Figure '7 in which I are the inputterminals receiving the total picture currents, 2 is an inductiveimpedance and 3 a condenser. Low frequencies pass more easily throughthe circuit containing the impedance 2 than through that containing thecondenser 3, and the picture frequencies pass more easily through 3 than2. A transformer l delivers the control frequency at the terminals Sfand the picture frequencies appear at the terminals 6. This arrangementdoes not provide complete separation, for this is not requiredspecifically, but at the terminals 5 the percentage of low frequency isgreatest, and at 6 least. Many arrangements are possible and are wellknown .in usual electrical practice, and there is no need actually tohave any separation. This will be understood from Figures 3 and 4. Thehump due tothe picture detail in the control frequency shown in Figure 3will tend todisplace the phase of, synchronization towards the hu p,bill in AXB C wherein A is the number of complete pictures per second,and B and C any whole numbers, such that B is not a multiple of C. Cgives the number of complete pictures over which the effect of thecontrol frequency is zero. visable that C be as small as possible, viz.2, in order that there be as little flicker as possible in thereproduced picture. a

When

the control frequency affords a means of securing completely automaticphasing or framing of the picture, being particularly convenient in os-.-cil1atory scanning. Such a frequency may also be used forsynchronization, provided it is truly sinusoidal rnd the synchronizingdevice responds correctly to the variatidn, for it is, less susceptibleto disturbing influences such as picture detail I and line frequencies,and will maintain synchronism for a longer period than is possible withhigher frequencies.

The manner in which control frequencies are balanced out will be betterunderstood from Figures 8-13.

Figures 8 and 9 show the appearance of a control frequency of /2 of thepicture frequency in alternate complete pictures.

Figure 8' shows the appearance of a received picture, say during thepositive half cycle ,of the control frequency, and Figure 9 shows thesame during the negative half cycle,.i. e. the appear ance of twosuccessive complete pictures. The shading of the pictures variessinusoidally from top to bottom, a half cycle being shown at the side.As these pictures alternate in their sequence, and in televisionapparatus appear in such rapid succession that individual picturescannot be detected, the picture of Figure 8 may be considered assuperimposed on the picture of Figure 9, so that the shadings are added,with the result that the picture shown in Figure 10 is obtained. InFigure 10a it is shown how the addition of the positive and negativevalues produces a constant mean value. This cancellation takes placeindependently of the phase relation of the control frequency to thepictures, and is unaffected by, nor does it affect the picture details,which appear just as though the control frequency was not present.

Figures 11 to 13 illustrate the case when the control frequency is i Thepicture has four strips so that the frequency is comparable with thestrip frequency. Figure 11 shows one picture and Figure 12 the next one.

'The curves in Figures 11a and 120. show the shade It is adconstantvalue shown as the dotted line. The curves shown in the strips show theshade value in the same strip, unidirectional scanning being presumed.

5 Similarly any control frequency, provided it bears a correct relationto the complete picture frequency, will cancel out in the picture asseen, and any number of frequencies may be superposed as described tothe picture currents. Pro- 10 vided each has a correct relation to thepicture frequency, none will appear in the picture.

Figures 14 to 1'7 show forms of filters according to this inventionwhich permit the control frequencies to be produced and be balanced inthe 1 picture received.

Where filters are employed in the transmitter, receiver or both, thecontrol frequency must be a multiple of the picture frequency unless thefilter is movable. In Figure 14 the frequency is four 20 times thepicture frequency, the picture being scanned in four horizontal strips.If the picture had any other number of strips the control frequency persecond would be that number multiplied by the number of pictures persecond. In

25 Figure 14 is shown a filter having a sinusodial variation of shadefrom left to right, but no variation from top to bottom; the scanning ofthe strips is effected horizontally. Obviously when the filter issuitably placed, preferably in the focal 30 plane of the image at thetransmitter, the picture currents are modulated by a control frequencyproduced by the filter. -A filter as shown in Figure 15, which is anegative of Figure 14, is

used at the receiver so that the effect 'of the control frequency isbalanced out of the'picture. The filter method of producing a controlfrequency is very simple and yet very effective. Figures 16 and 17 showthe transmitter and receiver filter for the same frequency as in Figures14 and 15, but the phase is displaced through 90. The filters as shownshould preferably be of double width so as to cover two cycles insteadof one and thus enable the receiver filter to be ad- 45 shown in Figure18.

- It will be seen that such a filter may be the same, irrespective ofthe number of strips in the picture. The frequency will always be thepicture frequency multiplied by the number of strips. The filter mayalso be used with the variation at right angles to the strips to producea control frequency equal to the picture frequency, which frame thepicture. In this case, the control device shouldbe polarized so thatthere will be only one position in which the picture will frame.

Moreover, two such filters may be used at right angles, either separateor combined in one filter, one variation being parallel to the directionof scanning and the other one at right angles thereto, whereby twocontrol frequencies are produced, one for automatic framing and theother or synchronizing.

In Figures 14-17 the filter has one cycle 'per (35 picture width, but itis possible to arrange that when a picture of a definte number of stripsis to be transmitted any number of complete cycles in the whole pictureis obtained. In the latter case the filter should only be used withpictures hav- 7 ing the number of strips for which the filter is made.Such a filter is shownin Figure 19, having four strips and cycles percomplete picture,

directional.

For oscillatory scanning filters must be more a,oee,aaa

carefully made and used, the general rule being justed to balance outexactly. Such a filter is i may be used automatically to adjust thephase or the scanning being again assumed to be unithat there, must be awhole number, or a whole number and a half, of complete cycles perswing, and if the scann ng during a swing is not at constant speed, thenthe filter must be so made that the frequency produced is constant. Theend of a swing should be always at a maximum or minimum shade value,otherwise there is a phase reversal of the control frequency producedwith alternate swings in scanning.

- may be effected by applying the same frequencies to control theintensity of light falling on. a photographic surface placed preferablyin a position where the image is formed or seen with the apparatus whenreceiving or transmitting actual pictures. The photographic surface isexposed for a sufficient length of time to attain the required density.When this photograph is formed it may be used as a master print, fromwhich any number of copies can be taken by contact printing, or anyother process.

Stops and apertures may be used to produce control frequencies, and toremove them from a received picture wherever a beam of light has atraversing or angular movement. An arrangement is shown in Figure 20illustrating the arrangement at a television transmitter using a Nipkowdisc.

The Nipkow disc I has formed on it the image to be transmitted. Asection of the image throws light through the scanning aperture 5forming a divergent pencil of light, which the condenser lens 9 focusseson to a photo cell Ill, in such a way that as 5 is moved to the positionII, there is no movement of the spot of light at the focus of 9. Anaperture I2, the size of which depends on the amount of variationrequired, is suitably placed between 9 and Iii. As shown, only half ofthe light from the aperture at 5 passes through I2, and as 5 movestowards II, the amount of light passing through I2 in},

creases to a maximum when 5 is midway between,

the positions 5 and II, and with further mo 7 ment towards II, itdecreases until at again half the maximum value. With a suitableaperture at I2 and a suitable shapejof holes in swings the reflectedspot of light moves between I the points I6 and II. In the extremepositions of swing the light passing through the aperture I5 is reduced,and is a maximum midway between I6 and I1. The result is the same asthat obtained with the filter according to Figure 15.

In Figure 22 light from the source I3 is reflected by the oscillatingmirror I4, the reflected spot moving between the points I6 and II. The.

stop I8 is of such a size andsopositioned that the light is maximum atthe points I6 and I1, and minimum midway -between-- them at .the,

point I9. The effect is therefore. the same :as

in the case of the filter according to Figure 14.

It will be seen that a side displacement of an aperture or stop willgive the same effect as the filters according to-Figure 16 or 17. tipleapertures or stops may be used to produce higher frequencies providedthat the pencil of light at the aperture or stop is of suitable size.-The effect is always similar to that of the filters above described.

It will also be seen from Figure 20 that two control frequencies may beproduced by an aperture or stop. This will be described with referenceto Figures 23 to 25.

In Figure 23, the aperture 20 has a suitable form dependent on the typeof scanning. The

extreme frame of the area over which the pencil of light moves in theplane of the aperture is indicated by the points 2|, 22, 23. Suppose thestrips of the picture to be parallel to 2l-22 it will be seen that themovement of the pencil of light in that direction will produce a controlfrequency equal to the strip frequency as described with reference toFigure 20. As there is also a slower secondary scanning in the direction2322 another control frequency is produced equal to the frequency ofcomplete pictures. The first frequency may be used for synchronization,and the second one for the automatic phase adjustment.

Figure 24 illustrates an arrangement similar to that shown in Figure 23,exceptthat a stop 24 is used instead of an aperture, the frequenciesproduced having a phase displacement of 180 relatively to those producedaccording to Figure 23. The arrangement may therefore be used in areceiver when an arrangement as shown inFigure 23 is used in thetransmitter, or vice versa, in order to eliminate the controlfrequencies from the received picture.

Figure 25 shows an arrangement of a stop 25 and apertures 26, 21, 28,29, 30, 3|, which likewise produces two frequencies, one equal to thepicture frequency and the other one double the strip frequency. Thecross section of the pencil of light should be approximately the same asthat of the apertures. Obviously such arrangements are possible whichenable other control frequencies to be obtained, exactly as is possiblewith filters.

The methods hereinbefore described may be applied to all systems ofscanning and assembling. How this can be effected will be readilyapparent, as regards mechanical systems. In the case of cathode rayscanning the application is not so apparent, and such arrangements willnow be more clearly explained.

As the control frequencies are mixed with the picture currents, and thecathode ray has practically no inertia, picture impulses will completelyupset the action of the control frequencies in the receiver. For thisreason it is necessary to impart to the cathode ray some artificialinertia,

or produce some equivalent effect so far as the movements of the ray areconcerned, so that impulses of short duration, such as from picturedetails, will not seriously disturb the posi:- tion of the cathode ray.This is most conveniently ensured by the use of electrical means, suchas inductances and condensers, though intermediary electro-mechanicalmeans may be used.

A typical arrangement is shown in Figure 26. wherein the'whole of thereceived currents are applied to the terminals 32. The actual picturedetail freqencies are takenfrom terminals 34,

Mul-

through a condenser 33 which offers little impedance to thosefrequencies, but a great impedance to the control frequencies requiredfor scanning. The inductances 35 and 36 offer a high impedance to thepicture detail currents, and a low impedance to the control frequencies,so that the latter pass through the primary of the trans former 31. Thesecondary of the transformer 31 supplies only very weak picture detailcurrents, but the control frequencies are strong. The higher controlfrequency is taken to the deflecting plates 38 of the cathode ray tubethrough a condenser 39 which offers little impedance to them, but highimpedance to the low control frequency. The inductance 40 offers a highimpedance to the higher control frequency and a low impedance to the lowcontrol frequency, so that the latter passes through the primary of thetransformer 4 I. The secondary of the transformer 4' I .supplies astrong low control frequency, a weak high control frequency, and veryweak picture detail frequencies. The two latter frequencies are furtherreduced in strength by a by pass condenser 42, offering a high impedancetothe low control frequency, which is applied strongly to the deflectionplates 38' of the cathode ray tube. A by pass condenser 44 may beconnected across the secondary of the transformer 31, which condenseroffers a low impedance to picture detail frequencies but a highimpedance to the other frequencies. An inductance 43 is connected in thecircuit of the deflecting plates 38 to prevent picture detailfrequencies from being applied at high strength to the said plates. Thisarrangement may be modified in many respects and use may be made ofnumerous other similar arrangements well known in practice.

, The effect of these arrangements is that strips of the picture arepractically undisturbed by detail frequencies, and only. slightly by thehigher control frequency, which may be an advantage for avoiding orreducing as much as possible dark bands between strips. The positions ofthe picture details in a strip are only very slightly disturbed by thestrength of those details, and in any case the extent of any possibledisplacement is such that the eye cannot detect it in the picture. Withelectro-mechanical methods, the control frequencies instead of beingsupplied direct to the deflecting plates 38 and 38' are supplied tosynchronous rotating orv oscillating devices, which generate or controlcurrents producing control frequencies independent of the currentsscanning method. The filter decreases in shade value from 44 to 45 inaccordance to the curve 46 which has a constant slope from 41 to 48. Inscanning in the direction from to 45, the picture currents are modulatedas shown in Figure 29, and such a form of current variation in thereceiver when applied to the deflecting plates will cause the spot ofcathode rays to move slowly at a constant speed from one side to theother, and then at the end of the movement to jump quickly back again,the process being then repeated.

Figure 27 will give only one scanning, but two quency of Figure 31. Onepart of these changes filters as shown in'Figure 2'larranged at rightangles to one another will give the double scanning. .The two filtersmay be combined in one, such as is shown in Figure 28, wherein the shadevalue decreases continuously at a constant rate from 44 to 48diagonally. Actually Figure .28 shows the same filter as Figure 27,- butthe shading has been rotated so that maximum variation of shade is alongthe diagonal. Suppose the filter according to Figure 28 to be scannedrapidly from 44 to 45, and the secondary scanning to be from 44 to 4|,then the filter will be scanned in strips, the number of which willdepend on the number of scannings from 44 to during one scanning from 44to 41. The resulting modulation of I the picture current will be asshown in Figure 30, if there are four scannings from 44 to 45 during thescanning from 44 to 47.

The points 49, 50 and Bishow the end of one complete scanning and thebeginning of the next, and the points 49, 52, 53, 54 and 50 thebeginnings of horizontal scannings and the ends of those preceding them.

Obviously there is a combination of two variations as shown in Figure31, viz. one of high frequency marked by the points 49, 52, 53, 54, 50,55, 56, 51, 5t, and the other one of low frequency from 49 to 50 andfrom 50 to 5!, the former being four times greater than the latter.

If the modulation according to Figure 30 or the two separate variationsaccording to Figure 31,

are present in the received picture, they can be eliminated by using afilter similar to that shown in Figure 28 but reversed, that is to say,with the shading decreasing from 48 to 44.

The combined control variations according to Figure 30 cannot beusefuilyapplied direct to the cathode ray tube, and so the components shown inFigure 31 must be separated before they are applied to the respectivedeflecting plates in the tube. A convenient way of doing this is shownin Figure 32. The picture currents modulated by the control currents aresupplied to the input terminals 5a, the impulses being applied to thegrid of the thermionic valve 68, through a resistance 59. Therefore thecurrents appear in full strength in the .anode resistance ti. The gridof the valve 62 is in parallel with the valve but is fed through aninductance or resistance 63 which offers a high impedance to picturedetail changes, a condenser 64 assisting in by-passing the picturedetail changes, so that the latter are considerably reduced at the gridof the valve 62. The valve taoperates in a similar way, except that theinductance or resistance 6E5 has a high impedance, and the condenser M alow impedance to picture detail changes and to the higher frequencychanges shown in Figure 31,; consequently the low frequency changes aregreatest at the grid of the-valve 55. The resistances 68 and 69 allow acurrent to flow through I53 and respectively so that the latterresistances are operative. The low frequency change in Figure 31 isgreatest in the anode resistance Iii of valve 65. The tapping on 10allows part of the voltage to be passed through the resistance ii and beapplied to the grids of the valves 66 and B2. The phase of the currentsso applied to the valves 60 and 62 is opposite to the phase they receivefrom the terminals 58 and consequently with correct adjustments the lowfrequency changes of Figure 31 will balance out from the currentsapplied to the valves 60 and 62. Therefore the changes of currentthrough the anode resistance 12 ofthe valve 62 correspond to thehigh'freflows through the resistance 18 to the grids of the valves andBI, and because of the opposite phase it balances out that frequencyfrom the anode currents of those valves. As the-low and.

high frequency changes of Figure 31 are balanced out in the valve 60,the current changes in the anode resistance correspond only to thepicture detail changes, and a part flows through the resistance 14 tothe grids of the valves 82 and 66 balancing out those changes in thecurrents of these valves. The final result is that the valve 60 passesonly picture detail changes, the valve 62 only the high frequencycontrol changes, the valve 85 only the low frequency control changes,and these may be taken independently from one another from the terminals15, 16 and 1'! respectively. Ii the terminals 16 and 11. are connectedto a cathode ray receiver, unidirectional scanning is obtained incorrect synchronism and phase with the transmitter. The arrangement ofvalves in parallel with interlocked negative reaction as It is alsouseful as a means of eliminating control frequencies from the receivedpicture. The separation, though not absolutely complete, may be madevery good with only a small percentage of the undesired frequenciesappearing, if adjustments are correctly made. Many modifications arepossible, but the characteristic of all of them is the negative reactionof circuits on other circuits in order to eliminate the frequencies ofthose circuits from the one on which they react; one circuit reacts onall other circuits or on those which require complete elimination of thefrequency of that circuit.

Another method of cancelling control frequencies froma received pictureconsists in arranging one strip of a picture to compensate the next. Inthis case no filter is required at the receiver but the size of thescanning aperture or light spot is such that it covers two or morestrips. Generally if more than two strips are covered the detail isimpaired. The method will be explained with reference to Figures 33 and34. Figure 33 shows a received picture of eight strips withoutcompensation-and Figure 34 shows how one strip compensates half of eachof two adjacent strips. The control frequency shown is half the stripfrequency. It will be seen that the of scanning being it, 33, is. As tilis negative relatively to it and it and lies over them, it compensatesthe latter strips to give an even shade.

The relation of the control frequency to' the line frequency for usewith this method is A B+C, wherein A is the line frequency, B any numbernot a multiple of C, and C the number of lines of the picture in whichcompensation is complete. The scanning aperture or spot must beincreased C times in the direction of the width of the strip. In Figures33 and 34 6:2 and 3:1. Scanning by an aperture twice the width of astrip of the picture is known from the previous invention described inBritish Patent No. 218,766, but the application for the purpose ofeliminating control frequencies from a received picture is new vTheiinvention as above described has been accomplished with stationaryfilters, apertures and .stops, and by what may be termed staticarrangements. There are however many possible methods using movingfilters and the like. Generally these will vary with the differentsystems of scanning, although the principles are the same. For instancein the case of a Nipkow disc a filter may be attached to the disc, theshade of the filter changing with the angle, so that one or morecomplete cycles are produced per revolution. The highest number ofcycles should preferably not be more than a quarter of the number ofholes required to scan one complete picture, otherwise'the frequencyproduced will deviate considerably from the sinusoidal form. Figure 35shows such an arrangement, giving one cycle per-revolution, which maytherefore be used for the automatic adjustment of the phase. A similarfilter used in the receiver, displaced through 180", eliminates thefrequency from the picture. Naturally, the filter need not becontinuous, allthat is required being that the shade covering theapertures varies around the disc as shown in Figure 35.

An alternative method which produces the same result'consists in makingthe apertures of varying area, preferably by varying the size in' thedirection of scanning, so that the light passed by the respectiveapertures shall be the same as would be obtained in Figure35. Thereceiver would of course be modified in the opposite sense, i. e. thehole in the receiver disc having the maximum area would correspond tothe one with minimum areain the transmitter, and the holes in the twodiscs would have the same relative position in the spiral.

The filters may be separate from the scanning member and may move at aspeed different from that of the latter so that the control frequencyproduced is dependent on the scanning and the speed of the filter. Whenthe scanning is unidirectional it is advisable to move the filter in thesame way, and with oscillatory scanning the filter should alsooscillate,',otherwise irregular frequencies may be produced.

The filters need not move in-the scanning directions, but may bearranged to move at an angle thereto, so that one filter can produce twocontrol frequencies at the same time by adiust-. ing the angle ofmovement in correct relation to the two scanning directions.

Moreover, the points of even shade value in the filter may be at anangle to the direction of scanning, i. e. the filter may be made such asshown in Figure 15 and be slightly displaced angularly when it willproduce the effect of the filter shown in Figure 11. This appliesequally tostationary as well as to moving filters.

Apertures and stops, which as has been explained above are theequivalents of filters, may also be moved in a similar way.

The control frequencies at the transmitter may be produced by ordinaryelectrical methods, for instance by separate generators and the like,the scanning being suitably synchronized therewith, or the generatorscoupled to the scanning arrangement. For instance, a generator may beprovided on the shaft of a Nipkow disc. The modulation of thephotoelectric current may also be effected by known electrical .means,such as thermionic valves.

In the receiver use may be made instead of the filters, apertures andthe like of generators coufrequencies for the neutralization of thecontrol frequencies in the picture currents, or their elimination fromthe picture.

Another way, of eliminating unwanted frequencies in a received pictureconsists in the modulation of light, for instance by such devices, asKerr cells, the modulation being such as to balance out the unwantedfrequencies. In light control devices using polarized light, additionalpoiarizers and analyzers need not be used; for instance more than oneKerr cell or when using the Faraday rotation of polarized light morethan one coil may be used in tandem or side by side between the crossedNichol prisms, one giving the normal light modulation required for thepicture, and the other or others a modulation at the unwantedfrequencies, but with a light variation which balances out the unwantedfrequencies from the picture.

In the case of the Kerr cell, the cell may be provided with additionalelectrodes for this purpose. There may be one or a pair of addedelectrodes for each unwanted frequency, although this is not necessary,since all such frequencies may be applied to one, or one pair of addedelectrodes. Figures 36 to 43 show a number of different arrangements ofsuch Kerr cells,only the electrodes being shown.

In all the arrangements, light passes through in the'direction of thearrows, and in Figures 36 and 37 it may pass in any direction in theplane of the arrows. In the arrangements shown in Figures 36 and 42, theunwanted frequencies are applied, say, to the pair of electrodes 8| andthe picture frequencies to the pair 82. In the arrangements according toFigures 37 and 40, 83 is acommon electrode for both frequencies, theunwanted frequencies being applied to one of the other electrodes 88 andthe picture impulses to the electrode 88. In Figure 38, the unwantedfrequencies may be applied to the electrodes 88 and 81 and the pictureimpulses to the electrode 88 as one pole and to the electrodes 88 and 81as the other pole, a resistance, inductance, or two condensers in seriesbeing connected across 88 and 81, and the impulses supplied to 88 beingapplied to a middle point of the'shunt across 86 and 81. In Figure 38the two kinds of currents may of course be applied in the opposite wayto the electrodes.

In Figure 39, the electrodes 88 and 88, 88 and 82, 82 and 8| and 8| and88 have shunts across them as mentioned in connection with Figure 38.The one kindof impulses are applied to the midpoints of theshunts 88 and88, and 8| and 82 and the other kind to the midpoints of the shunts 88and 82 and 88 and 8|. The arrangement according to Figure 30 maybe usedalso'without shunts, one kind of impulses 'being applied to 88 and 82and the other to 88 and 8|.

In Figure 41 the connections may be as in Fig-. ure 39 although theeffect is somewhat different. In Figures 38, 39, and 40 the plane ofpolarization lies at' an angle of with respect to the planes of thegaps, and in Figure 41 to the plane of 88 and 82. .This angle isgenerally the best for 9. Kerr cell. In Figures 36, 42 and 43 one kindof impulses is applied to the electrodes 8| and the other kind to theelectrodes 82.

When using modulated neon lamps or similar devices, an additionalelectrode may be provided, to which the unwanted frequencies areapplied,

, transmission, similar methods maybe used, the

picture impulses being modulated by, or mixed with, one or more controlfrequencies, and the frequencies being balanced out fromthe receivedpicture by one or more of the methods hereinbefore described. In themethods in which the cancellation takes place in two or more com,- pletepictures, two or more pictures must be superimposed in order that thecancellation be effected. The other methods are preferable in picturetelegraphy.

Where pictures are transmitted as modulations of a carrier frequency,improvements are secured by using this invention, since there is no needto rectify, the received modulated frequency being suitably applied tovary directly the intensity of a source of light and a compensatingmethod being used to balance out the carrier frequency. Except whenelectrical compensation is used, the relation of the carrier frequencyto the picture frequency must be as above described in connection withthe various methods.

It has been stated above that the control frequencies are sinusoidal,but this is not essential. The requirements are that the controlfrequencies should be continuous and regular as regards periodicity,wave form, phase and amplitude,

since these are the characteristics which make it possible to cancelthe-m out from the received picture, i. e. to separate'fromthe picturethe currents which are wholly irregular. Even when frequencies occur inthe picture currents which are equal to the control frequencies, theywill not affect the cancellation, unless they are absolutely as constantand continuous as the said control frequencies. When filter compensationis. used, such frequencies in the pictures may be as constant andcontinuous as the control frequencies without the received picture beingaffected. Aperture and stop compensation is of course equivalent tofilter compensation When compensation takes place in succeedingpictures, frequencies equal to control frequencies cannot occur inordinary practice, for if they could, movement of details in a scenemust be such as to produce a change of phase of the correspondingpicture impulse in the succeeding picture, which means such a rapidmovement in the scene that it could not be seen by the eye, or such astructure of the scene combined with a precise movement, as does notoccur in an ordinary scene, except very rarely. The same remarks applyto methods where compensation occurs automatically in succeeding stripsof the picture. 4

In electrical methods of compensation of the control frequencies, equal.and constant frequencies produced by picture details are not disturbed,provided the amount of compensation is maintained constant with a fixedvalue corresponding to the control frequencies only.

Generally true sinusoidal control frequencies are most satisfactory,more particularly with unidirectional. scanning, for departures fromsinus form obviously produce a number of sinusoidal frequencies, whichare more likely to disturb the received picture. Filter compensation ispreferable where the wave form is not sinusoidal.

With oscillatory scanning, however, a wave.

corresponding to the form of a full wave rectiends. This occurs almostwholly in the receiver and may be removed by thefilters or aperturesabove described. The effect is shown in Figure 44, where the ordinateY,represents intensity oi illumination and the abscissa X, time, 93-94being the period of one swing. It is seen that the curve has the form ofa full wave rectified sinusoidal frequency. Let us assume that thepicture currents in the transmitter are modulated, or have added tothem, a frequency such as shown by the dotted curve. If the effect ofthe frequency applied to the picture currents in the received picture isequal to the variation due to the oscillatory scanning in the receiver,and shown by the full line, then the picture appears to be evenlyilluminated, since the applied frequency is cancelled out of thepicture. by the scanning in the receiver. Nevertheless that frequencymay be used in the receiver in order to synchronize and adjust the phaseof the oscillatory scanning. Naturally, a control frequency of the formshown may very easily be produced in the transmitter, by using a filterof the form shown in Figure 1.5. Further, the filter may be such thatthe control frequency is perfectly cancelled out of the receivedpicture, since it can be produced photographicallyby the oscillatoryscanning. In Figure 44, the period 95-93 represents a swing of thescanning device in one direction and the period tit-Edits swing in theopposite direction; therefore the position oftfi in the picture is thesame as that of 9d, and a stationary detail in the picture which isscanned equally in two successive swings will produce the hump 96 in thedotted curve in one swing, and a corresponding hump 911 in the nextswing. As the control frequency shown by the dotted curve is used,todrive or control the oscillations of the scanning device, 98 will tendto advance the one swing, and 971 will likewise tend to retard the nextswing, so that over two swings the disturbance is zero, and in any caseso rapid that the eye cannot detect it in the received picture; if theinertia, or its equivalent, of the oscillatory scanning device isadequate no disturbance will occur.

Another method of applying control frequencies to the picture currentsin such a manner that they shall not appear in thereceived pictureconsists in adding to the picture currents, or modu at ng them with, achanging frequency of constant amplitude, the period of change being thecontrol frequency, and sinusoidal. This will be better understood fromFigure 45, wherein the curve 98 represents a sinusoidal variation andthe curve 99 a rectified sinusoidal frequency. The Y axis gives thefrequency and the X axis represents time. The curves show the change offrequency with time, the actual frequency being better seen in Figure 46for the curve 98. As shown in Figure 46 the amplitude is constant,although it may also be modulated. In the receiver the frequencies ofFigure 46 may be cancelled out of the picture by an appropriate method,which depends largely on the production of frequencies in thetransmitter. For instance,

el mination of frequencies.'

ill)

The received currents may be applied direct to the synchronizing device,if the latter consists of a non-polarized'magnetic, electrostatic, orelectrodynamic device of the dynamometer type; otherwise rectificationis necessary. The frequencies are passed through a circuit having twobranches, the one containing an inductance and the other one acondenser, as shown in Figure 4'1. This arrangement produces anamplitude modulation of the frequencies of opposite phases in the twobranches. I00 are the output terminals and I0! are rectifiers. If norectifiers are used, the primary of the transformer I02 is replaced bythe coils or the like of the synchronizing apparatus. The invention maybe used with, equal advantage in picture telegraphy and be readilyapplied to the various systems. When a cylinder is employed on which thepicture is attached or received, and use is made of filters, the lattermay also be of cylindrical form, or made" flexible so that they can bebent to therequired curvature.

It is an advantage that the filters be in contact with the picture, forwhich purpose use is preferably made of atransparent cylinder such asglass, with the filter permanently provided thereon, the picture in' thetransmitter, and thephotographic surface in the. receiver being placedinside or outside the cylinder according to the general construction ofthe apparatus, so that it is in contact with the filter. As will beunder-' stood, the filter will remain in the correct position withrespect to the phase adjustment and the synchronizing devices;

The methods preferred for picture telegraphy are those in'which thecompensation is effected by means of multiple electrode Kerr cells, orin which it takes place automatically in two strips of the picture,using the double size aperture or spot of light. Two control frequenciesmay be used, one corresponding to the number of revolutions of thecylinder, or to half. thereof, and the other one which is higher and isused to control the speed of the driving motor.

It is to be understood that theinvention is not limited to the'examplesand details hereinbefore described, asthey may be modified according torequirements; without departing from the spirit of the invention.

What I claim is:--

1. A method of television, picture -telegraphy and the like, consistingin adding uninterrupted control frequenciesto the picture currents,transmitting them simultaneously and at the same time as the said.picture currents, using the said control frequencies in the receiverfor synchronization, phase adjustment or framing and producing in thereceiver a counter effect equal to that of the control frequencies andthereby canceiling the said control frequencies from the receivedpicture as seen by the human eye.

2. A method as claimed in claim 1, wherein the control frequencies areadded to the picture currents simply by superposition. I

3. A method as claimed in claim 1 wherein the control frequencies areadded to the picture currents in the form of an amplitude modulation ofthe latter.

4. A method as claimed in claim 1, wherein the control frequencies areadded to the picture currents in the form of a frequency modulation of acarrier frequency.

5. Amethod of television, picture telegraphy, consisting in adding anuninterrupted control frequency to the picture currents, transmitting itat the same time as the said picture currents,

using the said control frequency in the receiver for synchronization andthe said control frequency being a fractional multiple of the frequencyof the picture scanning so that there is a change of phase of theeffects of the control frequency relative to the picture in a successionof picture scannings, which produces in the receiver an oppositioncontrol frequency effects in a group of successive picture scannings sothat the effect of the control frequency, due to the persistence ofvision, is zero in that group as seen bythe human eye.

6. A method of television, picture telegraphy, consisting in adding anuninterrupted control frequency to the picture currents, transmitting itat the the same time as the said picture currents, using the saidcontrol frequency in the receiver for synchronization phase adjustmentthe said control frequency being a fractional multiple of the stripfrequency of scanning the picture so that there is a phase displacementof control frequency effects relative to the picture in the direction ofscanning in a succession of strip scannings which are caused to, opposein a group of successive strip scannings 'by scanning at the receiverwith afractional overlap of strips, so'that the eifect of thecontrolfrequency, due to persistence of vision, is zero in that group as seenby the human eye;

7. An apparatus for television and picture telegraphy, comprising incombination with a transmitter for the simultaneous transmission ofpicture currents and control frequencies, a receiver having shadedoptical means interposed in the path of light forming the picture in thereceiver for the purpose of cancelling out from the picture as seen theeflects of the control frequencies.

ST A method according to claim 1, consisting in using electrical ,meansfor suppressing some of the control frequencies from the picture screensin the receiver byselective electrical circuits and the application ofopposing frequencies equal to the frequencies to-be suppressed.

9. In an apparatus for television and picture telegraphy, in which anumber of control frequencies are added to picture currents and aretransmitted continuously and simultaneously with the said picturecurrents and in which the said control frequencies are used in thereceiver for synchronization and phase adjustment 1 a scanning deviceandalight control device which together produce the picture, the saidlight device having in'addition to the normal electrodes and controlmembers to which the received picture currents are applied, othercontrol members to which the control frequencies obtained by partialfiltering from the received picturecurrents are applied to produce avariation of light opposite and substantially equal to the variationcaused by thecon'trol frequencies added to the picture currentsappliedto thenormal electrodes and control members of the light control devicefor the purposeof cancelling the effect of the control frequencies inthe picture.

10. Television and picture telegraphy receiving apparatus adapted forthe reception of combined picture and control electrical oscillationsand in which a counter balancing effect is produced for the purpose ofremoving the unwanted effects of the control oscillations comprisings,ose,sss creased intensity and said picture oscillation is i of reducedintensity compared with said combined oscillations, an electro-opticalcontrol device having a plurality of pairs of controlling members, meansfor applying potential difleren'ces corresponding to said combinedoscillations across one of said pairs of controlling members and meansfor applying potential differences corresponding to said correctiveoscillation across another of saidpairs of controlling members.

11. Television and picture telegraphy receiving apparatus adapted forthe reception of combined picture and control electrical oscillationscomprising means for producing from said combined electricaloscillations, a corrective electrical oscillation in which said controloscillation is of increased intensity and said picture oscilla-l tion isof reduced intensity compared with said combined oscillation, means forproducing light varying in intensity in accordance with said combinedoscillation and light varying in intensity in accordance with saidcorrective oscillation and means for combining the two light variationsin such a manner that variations due to said control oscillationsubstantially cancel one another.

12. In apparatus for television and picture telegraphy-in which a numberof control frequencies are added to picture currents and are transmittedcontinuously and simultaneously with the said picture currents and inwhich said control frequencies are used in the receiver forsynchronization and phase adjustment. the combination with a scanningdevice of an alternator and means coupling the scanning device to thealternator to ensure synchronous working the frequency of the alternatorbeing equal to the received control frequency and means for applying theoutput of the said alternator to a circuit containing the receivedpicture signals-in opposed balanced relation to the control frequencycontained in the said picture signals, for the purpose of eliminatingthe effects of the con- 20 trol frequency from the reproduced picture.

GEORGE WILLIAM WALTON.

